The aim of the proposed research is to develop and validate the function of a flexible, multi-axial, lower limb prosthesis designed from high strength composite materials. The Advanced Biofidelic Lower Extremity (ABLE) Prosthesis concept utilizes a composite material bi-phasic and asymmetric stiffness ankle to improve function/improve durability over existing at a comparable weight and cost. Structurally, the use of high strength composites as the structural and elastic elements in the prosthesis will provide greater service life and lighter weight for the amputee as well as high design flexibility in the elastic element mechanical response. Functionally, the unique bi-phasic of the ABLE elastic elements more closely mimics the function of the triceps surae and tibialis anterior muscles, enhancing mid-stance stability while still providing for the dynamic elastic responds beneficial at heel strike and push. Studies indicate that a limb providing enhanced function will: 1. Allow the amputee to be more active in their daily lives and 2. Improve their perception of their prosthesis as compared to the amputees' baseline condition. The program: 1. Establishes the biomechanical design parameters-where the desired performance is more clearly quantified on the basis of non- amputee gait, 2. Translates the design goals into the physical geometry and materials properties of the device, 3. Assures satisfactory performance by analyzing the design using state of the art finite element analysis prior to fabrication and 4. Tests the fabrication device mechanically (static loading and fatigue cycling) on both a component and assembly level and by amputees' in a clinical environment. The limb will be evaluated in the design phase by a small group of amputees to: 1. elicit feedback and 2. Measure actual performance. Finding will be used as input for the subsequent design iteration. At the third and final iteration we will evaluate the ABLE Perosthesis in an extensive real-world testing using validated outcomes measures to assess the performance or the limb versus subjects' prescribed limb, using a statistically significant pool. These outcomes will be measured through long tern evaluation using previously validated objective measures of amputee function, the Step Activity Monitor and Prosthesis Evaluation Questionnaire.